JPH0665239B2 - Printed wiring board - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printed wiring board used in electronic devices and the like, and more particularly, to mounting a chip-shaped component and soldering for electrical connection with electrodes of the chip-shaped component. The present invention relates to a printed wiring board capable of solder-connecting a chip-shaped component and a soldering land by sequentially transporting a board having a land on its upper surface into a heating furnace for soldering and heating from the back surface of the board.

2. Description of the Related Art In recent years, as electronic devices have become smaller and lighter, thin printed wiring boards have been used and chip-shaped components have been mounted by reflow soldering.

Generally, in order to improve the solderability in the reflow soldering method for chip-shaped components, heat is dissipated by separating the soldering lands 22a and 22b of the printed wiring board 21 by another copper foil 25 as shown in FIG. Or by adjusting the temperature conditions and heating time settings for reflow soldering to the characteristics of each printed wiring board 21, or arranging the soldering lands 22c sideways with respect to the heating direction. Is done by.

[Problems to be solved by the invention]

However, in the printed wiring board having the above-described conventional configuration, even if the soldering lands 22a and 22b are made independent or the temperature condition and the heating time are adjusted, the soldering lands 22a of the component 26 are
Since and 22b are conveyed to the reflow furnace by a belt conveyor etc. and are heated and cooled in the order close to the heating direction, as shown in Fig. 5 which shows the temperature change of the soldering land in the reflow furnace. , Soldering land
A time shift occurs in the temperature drop region between 22a and 22b. This phenomenon will be described in more detail with reference to FIG. 5. When the printed wiring board 21 is conveyed into the reflow furnace, the soldering lands 22a start to be heated. Then, after the time t1, the temperature of the soldering land 22b starts to rise, and after the time t2, both the soldering lands 22a and 22b reach the soldering temperature Ta, the solder 24 melts and the electrodes of the chip-shaped component 26 and the soldering land 22a. And 22b are each soldered. Next, the printed wiring board in the reflow furnace
If the temperature of the soldering land 22a decreases as the temperature of the soldering land 22a goes down when the transportation of 21 continues to pass through the heating area and starts the cooling process, the temperature of the soldering land 22a decreases to Tb and the soldering land 2a
The solder 24 on 2a is in a semi-cured state. Meanwhile soldering land
Since 22b is still in the heating region in the reflow furnace, its temperature remains Ta even at time t3.
The solder 24 on 22b is still molten.

In such a state, the chip-shaped component 26 may behave abnormally. That is, the chip-shaped component 26 moves to the heating direction side on the substrate and short-circuits with other soldering lands, or the soldering lands at the temperature Tb as shown in FIG.
Due to the surface tension during the process of hardening the solder 24 on 22a,
There is a problem that the end portion of the chip-shaped component 26 is pulled up from the solder 24 on the soldering land 22b which is still in a molten state, and a so-called rising phenomenon (also called tombstone phenomenon) occurs.

In addition, a considerably large mounting space is required in order to arrange all the chip-shaped parts 26 sideways with respect to the heating direction,
In high-density mounting, there is also a problem that mounting efficiency is reduced.

The present invention solves the above problems, and an object of the present invention is to provide an excellent printed wiring board that can improve the soldering quality of chip-shaped components in the reflow soldering method.

Means for Solving the Problems In order to achieve the above-mentioned object, the present invention is a chip-shaped device having a plurality of electrodes mounted on a substrate by sequentially heating the substrate from the back surface thereof while the substrate is being transported into a reflow furnace. In a printed wiring board for reflow soldering in which components are soldered, a solder on the board corresponding to the frontmost electrode in the traveling direction in which a chip-shaped component having a plurality of electrodes mounted on the board enters the reflow furnace A copper foil land having the same shape as the soldering land is arranged only at the same position on the back surface of the mounting land.

Action According to the present invention, therefore, the soldering lands are provided only at the same position on the back surface of the soldering land on the substrate corresponding to the frontmost electrode in the traveling direction in which the chip-shaped component having a plurality of electrodes enters the reflow furnace. Since the copper foil lands of the same shape are arranged, the soldering lands on the substrate are heated from the back surface in order from the traveling direction and are reflow-soldered, and at this time, the frontmost part that enters the reflow furnace is first. The entire copper foil land provided on the back surface of the soldering land is heated, the entire copper foil land has a uniform temperature, and the heat retaining effect is good, so that it can be a uniform and long-term indirect heat source. Therefore, the entire soldering land on the opposite side is uniform and
Since the heated temperature can be maintained, the time lag during cooling in each soldering land corresponding to multiple electrodes is reduced, and the hardening of the solder during the cooling process is almost the same, and the movement of the chip-shaped parts The rising phenomenon disappears and the soldering quality improves.

EXAMPLE A printed wiring board according to an example of the present invention will be described below with reference to FIGS. 1, 2, and 3. FIG. 1 is a partial perspective view showing the structure of one embodiment of the present invention, and FIG.
FIG. 3 is a partial cross-sectional view showing the same embodiment in a reflow furnace, and FIG. 3 shows a time relationship of heat received by a soldering land on a printed wiring board in the reflow furnace.

As shown in FIG. 1, soldering lands 2a and 2b made of copper foil or the like are provided on the upper surface of the substrate 1, and solder 3 is applied on the soldering lands 2a and 2b by printing or the like. A wiring pattern formed of a copper foil 4 is connected to these soldering lands 2a and 2b as required. Further, a copper foil land 5 which is a feature of the present invention is arranged on the back surface of the substrate 1 at the same position as the soldering land 2a, that is, at the same position as the back surface of the soldering land 2a through the substrate 1.
When the substrate 1 on which the chip-shaped component 6 is mounted is conveyed to a reflow furnace (not shown), the copper foil land 5 is firstly the reflow furnace among the plurality of electrodes 7a and 7b of the chip-shaped component 6. The soldering lands 2a corresponding to the electrodes located at the front position of the board must be provided only on the back surface of the same position on the substrate. That is, as shown in FIG. 2, the chip-shaped component 6 mounted on the upper surface of the substrate 1 has its electrodes 7a, 7b connected to the soldering lands 2a, 2b via the solder 4, and is transferred to the reflow furnace. , Soldering is connected by being heated. In this case, the substrate 1 is moving in the reflow oven in the direction indicated by the arrow, and therefore the soldering land 2a becomes the electrode at the front position which first enters the reflow oven, and the copper foil land 5 is used for this soldering. It is arranged only at the same position on the back surface of the substrate 1 corresponding to the land 2a. By taking such a configuration, when the printed wiring board according to the present invention is heated in the reflow furnace, the soldering lands 2a, 2b of the printed wiring board are sequentially heated from the back surface thereof in the traveling direction and reflow soldered. It At this time, first of all, the entire copper foil land 5 provided on the back surface of the soldering land 2a is heated, the entire temperature of the copper foil land 5 becomes uniform, and since the heat retaining effect is good, it can be a uniform and long-term indirect heat source. . Therefore, since the entire soldering land 2a on the opposite surface can uniformly maintain the heated temperature for a long time, the soldering land 2a which has reached the solder melting temperature Ta at time t4 as shown in FIG. Holds the melting state until time t5, while the soldering land 2b, which has started to be heated after the time t6 from the soldering land 2a, reaches the melting temperature Ta at the time t7, and then reaches the time t5 at almost the same time as the soldering land 2a. When the cooling process is started, the time difference of the cooling process in the soldering lands 2a and 2b is reduced, so that the hardening of the solder 4 in the cooling process progresses almost at the same time. As a result, the phenomenon that the chip-shaped component 6 moves or rises is eliminated, and the soldering quality can be improved.

Needless to say, the same effect can be obtained in a multi-electrode component such as a chip transistor having three electrodes or an IC.

As described above, according to the above embodiment, since the copper foil lands 5 having substantially the same shape as the soldering lands 2a are provided only at the same position on the rear surface of the substrate 1 of the soldering lands 2a of the substrate 1, the soldering lands 2a and There is no time lag in the cooling start time with respect to 2b, so that the movement and rising phenomenon of the chip-shaped component 6 can be eliminated.

EFFECTS OF THE INVENTION As is apparent from the above-described embodiments, the present invention provides soldering on a substrate corresponding to the frontmost electrode in the traveling direction in which a chip-shaped component having a plurality of electrodes mounted on a substrate enters a reflow furnace. Since the copper foil lands having the same shape as the soldering lands are arranged only at the same position on the back surface of the land substrate, it is possible to prevent the movement and rising phenomenon of the chip-shaped component during soldering in the reflow furnace, It is possible to significantly improve the soldering quality of the chip-shaped component.

[Brief description of drawings]

FIG. 1 is a partial perspective view of a printed wiring board according to an embodiment of the present invention, FIG. 2 is a partial sectional view of the printed wiring board for explaining a heating state in a reflow furnace, and FIG. FIG. 4 is a characteristic diagram showing the relationship between the temperature of each soldering land of the printed wiring board and time.
Fig. 5 is a plan view of a conventional printed wiring board, Fig. 5 is a characteristic diagram showing the relationship between temperature and time of each soldering land of the printed wiring board in the reflow furnace, and Fig. 6 is a diagram showing the soldering in the reflow furnace. 3 is a partial cross-sectional view of the same printed wiring board of FIG. 1 ... Board, 2a, 2b ... Soldering land, 5 ... Copper foil land, 6 ... Chip-shaped parts, 7a, 7b ... Electrodes.

Claims (1)

[Claims] 1. A print for reflow soldering, in which a chip-shaped component having a plurality of electrodes mounted on the substrate is soldered by being sequentially heated from the back surface of the substrate while being conveyed in a reflow furnace. In the wiring board, the same position on the back surface of the board of the soldering land on the board corresponding to the frontmost electrode in the traveling direction in which a chip-shaped component having a plurality of electrodes mounted on the board enters the reflow furnace A printed wiring board in which copper foil lands having the same shape as the soldering lands are arranged only on the above.